The invention described herein was made in the performance of work under a National Science Foundation grant identified by #CMS-9817695.
This relates to the determination of the content of one or more materials in a cement containing composition, such as the amount of chloride in a cement based composition.
The construction industry is interested in new techniques for nondestructive inspection of materials. Currently the techniques used are adequate in some cases, but may not be the case in others. One solution to this problem would be to combine one technique with others.
Chloride has been found to corrode steel members in reinforced structures. In solving this problem, early detection and close monitoring of chloride contaminated structures is essential in maintaining these structures. A nondestructive technique for determining chloride contamination would be beneficial.
Research has found that non-conducting materials, i.e. dielectric materials, can be analyzed by using microwave nondestructive testing (NDT) techniques. Microwave NDT techniques have been used to find surface and sub-surface degeneration in layered materials due to impact damage, and to measure the thickness of dielectric sheets. Such techniques also find unfilled spaces and air bubbles (locally and distributed) in dielectric materials, and are used to locate and evaluate disbond and delamination in multi-layered structures. Microwave signals can be used to measure the dielectric properties of a material. By knowing the dielectric properties of cement, aggregate, and sand, microwave signals can be used to measure the properties of the combined mixture. Also, this can be used to determine the curing rate and the presence of chemical reactions in the mixture.
Research has been conducted in this area. It has been found within recent years that this technique can be utilized for inspecting cement based construction composition. Near and far field techniques were the two main groups studied. The near and far field regions are based on the distance in which the sensor and the composition are separated from each other. Ground penetrating radars are an example of a far field technique and have been used successfully. Although it has been a success, there are still disadvantages to this technique. For example, repetitious calibration of the measurement equipment, the system spatial resolution, the inaccuracy of the data needed due to unwanted objects and the tedious signal processing needed to analyze the data are drawbacks. These are avoided when using the near field technique. The setback of operating in this region is that the electric and magnetic fields are very complicated to model.
The measured magnitude of reflection coefficient is shown to increase as a function of decreasing w/c ratio for cured cement paste. At first glance this seems inconsistent with the fact that higher water content should render a higher magnitude of reflection coefficient measured at a waveguide aperture. However, a closer look reveals that during the curing process water molecules bond with cement molecules, and some of the remaining free water evaporates. Thus, the water becomes less free and more bound over the curing time. Free water has much higher dielectric properties compared to those of cement powder, whereas bound water has similar dielectric properties to those of cement powder. In addition, higher w/c ratio specimens lose more of their free water to evaporation. Thus, the measured magnitude of reflection coefficient of these specimens decreases as a function of increasing w/c ratio.
The magnitude of reflection coefficient has been shown to be distinctly correlated to the w/c ratio of cement paste, and subsequently to its 28-day compressive strength (moist cured for 3 days in a hydration and thereafter in an air room temperature).
A simple expression predicting the microwave reflection properties of cement paste as a function of time has been obtained. Consequently, the w/c ratio of a cement paste specimen may be obtained by comparing two reflection coefficient measurements conducted several hours or a few days apart after the paste has been cast. In addition, it is possible to correlate the compressive strength of cement paste during curing to the measured microwave reflection properties (as a percentage of the 28-day strength).
A relationship between the standard deviation of the magnitude of reflection coefficient at higher frequencies and the s/c (sand/cement) ratio of a mortar specimen, has been established. Information on the w/c ratio of mortar specimens is obtained when the average value of the measurements is taken at relatively low microwave frequencies.
Mortar is a homogeneous dielectric mixture (even when measured at a frequency of 10 GHz). A simple dielectric mixing model has been obtained which predicts the constituent volume content of a mortar specimen. Consequently, the porosity (volume content of distributed air) of a mortar specimen can also be determined.
The statistical behavior of the microwave reflection properties of concrete as a function of w/c, s/c and ca/c (coarse aggregate/cement) ratios and the frequency of operation has been studied. It has been determined that the probability distribution functions of the measured magnitude of reflection coefficient of concrete, measured at high and low frequency bands, possess distinct and well-known distributions. At higher frequencies, the distribution is Gaussian whereas at low frequencies the distribution is uniform. With the use of the modifiable parameters in each of these distributions, the constituent volume distribution of a given concrete mixture can be determined from its scattering characteristics.
Similar to mortar, the results of the reflection property measurements indicate that the w/c ratio in concrete, and hence its strength, can be correlated to the average value of the magnitude of reflection coefficient measured at several independent locations on a specimen at lower frequencies (i.e., about 3 GHz). At lower frequencies the influence of aggregate size distribution is less on the measured magnitude of reflection coefficient than at higher frequencies since the aggregates electrically xe2x80x9clook smallerxe2x80x9d at lower frequencies.
Similarly, the standard deviation and the statistical distribution of the measured magnitude of reflection coefficient at higher frequencies is a function of the aggregate size and volume distributions. Hence, the constituent volume fraction and distribution of a concrete specimen may be determined at higher frequencies (i.e., about 10 GHz).
It has been shown that the cure state of concrete specimens, containing different w/c ratios and constitute makeup, can be unambiguously determined when making daily measurements of the magnitude of reflection coefficient.
It has also been shown that the w/c ratio of fresh concrete can be unambiguously determined independent of its s/c and ca/c ratios. This is an important finding since now an operator is capable of determining the w/c ratio of a batch plant concrete at the time of pouring.
It has been demonstrated that the extent of aggregate segregation in concrete placement can be evaluated using the statistics of the measured magnitude of reflection coefficient. This information can be easily obtained for concrete members such as walls and columns in which aggregate segregation may be an important practical issue.
Using an optimal frequency of operation, it has been effectively demonstrated that using a simple near-field and nondestructive microwave inspection technique employing an open-ended rectangular waveguide probe at 3 GHz (S-band) one can easily distinguish between empty and grout-filled masonry cells. In addition, a simple and extremely effective custom-built microwave inspection system has been designed and assembled for this purpose. This system has been successfully tested on a variety of masonry blocks.
Up to this point, the near field microwave NDT technique has been successfully applied to the inspection and characterization of cement based materials in several studies including detection of rebar in reinforced concrete; determination of variations in aggregate size distribution in concrete; determination of compressive strength and water-to-cement (w/c) ratio of hardened cement paste (cement and water); prediction of the microwave reflection properties of mortar (cement, sand and water) using a dielectric mixing model as a tool for obtaining the volume fraction of individual constituents of mortar; determination of the distributed porosity in mortar; determination of sand-to-cement ratios in mortar using the stochastic properties of its microwave reflection properties; and determination of the coarse aggregate volumetric distribution in concrete.
Concrete normally provides reinforcing steel with adequate corrosion protection. When steel is encased in concrete, a protective iron oxide film forms at the steel-concrete interface due to the high pH level associated with concrete. This film protects the steel from corrosion. However, the intrusion of chloride ions in reinforced concrete can destroy this protective film. If moisture and oxygen are present in the concrete, the steel will corrode through an electrochemical process. Once the steel begins to corrode, the concrete will deteriorate. This occurs because the byproducts of corrosion occupy a greater volume than the steel itself, which exerts a substantial stress on the surrounding concrete.
In accordance with the present invention, a determination is made related to the presence of at least one predetermined material in concrete or cement sample. In one embodiment, the material is one that is not normally included when the concrete is formed. For example, the material can be a salt that may include chloride. The salt may penetrate the concrete after it is formed. Alternatively or additionally, at least parts of the salt might have been included with the concrete when it was made. In one embodiment, the presence of the predetermined material is detected. Additionally or alternatively, the amount of the predetermined material is determined. Additionally or alternatively, a magnitude is determined related to the penetration of the predetermined material in the concrete, particularly the depth that the material might be found from the surface of the concrete.
An apparatus that can be used to make one or more such determinations includes a signal generating subsystem, a coupler subsystem and an analyzer subsystem. With regard to making one or more such determinations, the signal generating subsystem outputs microwave signals that are applied to the coupler subsystem. The coupler subsystem includes a transmitting section that carries the microwave signals to the concrete that is under observation or test. Reflected or returned microwave signals are generated due to the incidence of the transmitted microwave signals on the concrete sample. These are received by the receiving section of the coupler subsystem. These returned microwave signals are input to the analyzer subsystem, which makes the determinations related to the presence, amount and/or penetration associated with the predetermined material.
The analyzer subsystem includes at least one memory. The memory stores model information related to the predetermined material. In particular, the model information includes data or other information related to the predetermined material and one or more magnitudes of reflection coefficients. These are obtainable from the reflected microwave signals. They are useful in making the determinations related to the predetermined material. The model information is obtained based on measurements made using cement samples that were previously analyzed under known conditions. The model information that is obtained based on such testing and measurements can be presented in many different or related forms, such as an equation, a graph and/or a look-up table. The model information correlates the predetermined material in the concrete and associated dielectric property information (e.g., reflection coefficient magnitudes). Thus, when making determinations related to the predetermined material for the concrete under test or in the particular cement sample, the one or more reflection coefficient magnitudes measured using the cement sample are found and these determined magnitudes are used to make determinations related to the predetermined material, such as by use of a look-up table that correlates the determined one or more reflections coefficient magnitudes with formation related to the predetermined material of interest.
With respect to obtaining the data or other information to which the reflection coefficient magnitudes are to be correlated, certain steps are conducted associated with making measurements to provide such information. More specifically, a cured cement specimen is made or otherwise provided. The cured cement specimen may include some predetermined material or it may not. The cured cement specimen is located in a bath associated with known conditions. In one embodiment, the bath is a salt bath that has chloride as the predetermined material. The cured cement specimen is maintained in the salt bath for a desired or known time interval. The cement specimen is removed from the bath after the known time interval. It is allowed to dry. Then, one or more magnitudes of reflection coefficients are measured for this cement specimen. Later at different time intervals, one or more additional magnitudes of reflection coefficients are determined. This is continued until there is essentially no change in the measured magnitudes of reflection coefficients or such measurements are within an acceptable variation of each other.
Additional cured cement specimens are provided. For each of the cement specimens, the foregoing steps are implemented. For at least some of these cement specimens, they are placed in the bath having the predetermined material for different, known time intervals. Accordingly, measurements of magnitudes of reflection coefficients for other cement specimens are made after different time intervals related to how long the particular cement specimen remained in the bath.
In conjunction with obtaining model information related to the predetermined material in a particular cement sample, it may be desirable to further analyze the cement specimens after the one or more measurements of the magnitudes of reflection coefficients. In such a case, a cement specimen may have one or more sections removed therefrom. In one embodiment, a cylindrical cored section is removed from which a number of smaller in height cylindrical sections (slices) are severed. Subsequently, the cored portions are ground. The ground portions are subject to an analysis step involving an instrument, such as an electron microscope or the x-ray fluorescent machine, which can provide information related to the content of the predetermined material in the cement specimen. Such analysis can verify the accuracy of the measuring step, as well as provide information related to penetration of the predetermined material within the body of the concrete specimen from its surface.
Based on the foregoing summary, a number of advantages of the present are readily discerned. Information related to the presence, amount and/or penetration of a predetermined material in concrete can be obtained using model information. The model information can include chloride model information. The present invention is useful when the predetermined material is included with and/or becomes part of the concrete after it has been formed. Substantial and extensive testing is conducted to obtain the model information, particularly using a number of cement samples that have been cured and are subject to a bath having the predetermined material. Utilizing model information, such as chloride model information, the salt or chloride content of concrete can be monitored over time related to ascertaining currently existing properties of the concrete, such as whether its structural integrity is jeopardized by unacceptable levels of salt content.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.